Laser Doppler Velocimetry (LDV) has been utilized extensively in the past for measuring flow velocities by tracking particles seeded into the flow. The seeding requirement, however, is often undesirable and sometimes impractical (e.g., flames). In aerodynamic research, strong velocity gradients (e.g., shock waves, vortex flows) make it difficult for the particle to track the flow due to the large inertia of the particle relative to the molecules. The resulting particle lag represents a severe limitation to the application of LDV techniques at supersonic speeds. Seeding of particles into specific regions of interest is also very difficult. For example, seeding into the center of a vortex flow is hampered by the centrifugal forces experienced by the particles which render the vortex center void of particles. A new technique is presently being used to measure the flow velocity of the molecules directly using non-intrusive laser techniques. In the following, this evolving new technique for a molecular flow velocimeter will be described.
The concept utilizes the Raman scattering of light from specific molecules in the flow (e.g., N.sub.2, O.sub.2, H.sub.2). If the molecules are moving, the scattered light also exhibits a shift in frequency due to the Doppler effect. The shift in the Raman spectra can, therefore, be directly related to the velocity of the molecules. The intensity distribution in the Raman spectrum could also be used to obtain the temperature and density of the gaseous molecules. Spontaneous Raman spectroscopy (single laser) could in principle be used for this purpose, but weak signals and inadequate spectrometer resolution normally preclude such a measurement. The use of coherent, stimulated Raman spectroscopy, however, increases the signal by orders of magnitude and has a considerably higher resolution due to the inherently narrow line widths associated with lasers.
In one version of coherent Raman spectroscopy, called stimulated Raman gain spectroscopy (SRGS), two lasers interact with the molecules and the difference in frequency between these lasers is adjusted to be in resonance with a Raman shift exhibited by a specific molecule. Many geometrical arrangements can be envisioned for these two lasers. Two configurations, however, define the limits to their interactions with the molecules-namely, forward and backward scattering. In forward scattering, both laser beams co-propagate from the same direction; whereas in backward scattering, the laser beams are in a counter propagating configuration.
A typical set-up in SRGS involves a probe laser (usually a C.W. laser) and a pump laser (usually a high power pulsed laser). One of these lasers is tunable in frequency in order for the two lasers to come into Raman resonance with a particular molecule. Both lasers are tightly focused at the same point in space (typically 50-100 micron diameter) and the stimulated Raman interaction takes place only over a very small volume of space. In the SRGS set-up just described, a change (gain) in the probe intensity takes place only during the brief time (typically 10 nano-seconds) that the pump beam is present in the interaction volume. This change is sensed by focusing the probe beam on a detector and monitoring its output with high speed electronics. In both the forward and backward scattering cases, a double-ended configuration has been employed. In the forward scattering case, the detector is mounted on the opposite side of the interaction volume from the two lasers. In the backward scattering case, the detector is mounted on the table supporting the pump laser while the probe laser is mounted on the opposite side of the interaction volume. In the forward scattering case, the moving molecules "see" two laser beams whose frequencies are shifted in the same spectral direction and by about the same amount; hence there is very little Doppler shift (and breadth) by which to measure velocity. In the backward scattering case, on the other hand, the molecules "see" one laser beam shifted in one spectral direction while the other laser beam is shifted in the opposite direction. This results in a large Doppler shift (and breadth) which is considerably easier to measure. For N.sub.2 molecules, a Nd:Yag laser pump (532 .eta.m) and a dye laser probe (607 .eta.m) render backward scattering shifts fifteen times that observed in forward scattering. The ratio of Doppler shift to breadth, however, is actually the same in both cases, but the larger shift exhibited in backward scattering is easier to measure since it relaxes the stability and line width requirements placed on the lasers.
This concept for measuring molecular velocities has been demonstrated in the laboratory over the past several years. However, in both the forward and backward scattering cases, the double-ended configuration requires tightly focused laser beams and would be impossible to apply in severe vibrational environments, such as wind tunnels.
Thus, an object of the present invention is to provide a method and apparatus for making accurate measurements in a flow volume using SRGS that are unaffected by severe vibrational environments, such as wind tunnels.
A further object of the invention is to provide a method and apparatus for attaining overlap of two counter propagating beams emanating from the same side of a flow volume thereby providing accurate backward scattering measurements in the flow volume.
Another object of the invention is to provide a method and apparatus utilizing only two beams to produce both forward and backward scattering of light in a flow volume.
Other objects and advantages of the present invention will be readily apparent from the following description and drawing which illustrates a preferred embodiment of the present invention.